Modern-day semiconductor manufacturing utilizes optical lithography projectors that image mask patterns onto semiconductor wafers as part of the process of making integrated circuits. The optical lithography projectors include illumination systems (“illuminators”) that irradiate the mask with UV light because the smaller wavelength of UV light (as compared to visible light) allows for higher-resolution imaging. The optical lithography projectors utilize projection lenses (also called microlithographic lenses or optical lithography lenses), most of which include at least some glass lens elements. Because of the high demands placed on the imaging resolution over the image field, the projection lenses are designed for high performance.
A portion of the UV radiation (light) that passes through a given glass lens element interacts with the optical material. This includes light absorption whereby the absorbed light energy is converted to heat. The heat raises the local glass temperature, which changes the local index of refraction, thereby adversely changing the imaging performance of the projection lens. This effect is reversible, i.e., when the illumination terminates, the glass cools down and the imaging performance is restored. Thus, turning the illumination on (or off) gives rise to a thermal transient effect whereby the projection lens heats up (or cools down) until the temperature stabilizes. During this time, there is a thermally induced variation in imaging performance.
A second type of interaction between the UV light and the glass material changes the glass atomic structure and makes the structure denser, thereby locally increasing the glass refractive index and degrading the projection lens imaging performance. This effect is called “radiation compaction.” It is not reversible, and the attendant refractive index increase is proportional to the irradiance of the exposure light as well as the time-accumulated irradiance of the exposure light.
A third type of interaction is called solarization, which also changes the glass atomic structure and reduces the glass transmission. Like radiation compaction, solarization is not reversible and is also proportional to the light irradiance within the glass and the time-accumulated irradiance.
Consequently, when a projection lens is exposed to high irradiance UV light over an extended period of time, compaction and solarization effects can permanently damage the glass elements and result in significantly degraded imaging performance. These adverse effects are becoming more problematic as increasingly greater amounts of UV radiation and increasing large imaging fields are being employed, which lead to greater irradiance levels traveling through the glass elements. Further, increasingly higher degrees of imaging performance limit the amount of performance degradation that is acceptable.